The US State Department official, asking not to be named, agreed the negotiations "have been tough and very serious".

President Barack Obama and German Chancellor Angela Merkel also upped the pressure on Iran.

"Iran must make the decisions necessary to resolve several remaining issues," a statement said after the two leaders spoke by telephone.

Iranian President Hassan Rouhani called Thursday for the "unjust" sanctions choking the country's economy to be lifted as he wrote letters to the leaders of all six countries.

Rouhani also phoned the presidents of Russia, China and France and Britain's prime minister. The US official said Saturday Washington hoped this was "a sign that Iran is ready to make some of the tough decisions" needed.

But global powers insist sanctions will only be suspended, not lifted, and in a phased manner in case Iran violates the deal.

The UN has imposed several rounds of sanctions since 2006 aimed at stopping Iran from expanding its nuclear and missile programmes while EU and US sanctions since 2010 have targeted its oil exports and banks.

Iran's Insa news agency quoted an Iranian negotiator as saying that "solutions" had been reached in the talks over the US and EU sanctions but that the UN ones remained a problem.

"The UN sanctions... have to be lifted completely and immediately," the diplomat said. This is thought to be a no-go for the powers, all except Germany permanent members of the UN Security Council.

Kerry is under pressure to return from Lausanne with something concrete to head off a push by Republicans to introduce yet more sanctions, something which could torpedo the whole negotiating process.

The Republicans are concerned that by leaving some of Iran's nuclear infrastructure intact, as seems likely, the mooted deal will not do enough to prevent Iran getting the bomb.

This concern is shared by Israel, widely assumed to have nuclear weapons itself, as well as Saudi Arabia, worried by the prospect of any US-Iranian rapprochement with the West after 35 years of acrimony.